Fuel cells can continuously generate electric power by continuous supply of a fuel such as hydrogen and an oxidizing agent such as oxygen thereto. Unlike primary batteries such as dry batteries and secondary batteries such as lead storage batteries, the fuel cells each generate electric power at high generation efficiency without being significantly affected by the scale of a relevant system. In addition, the fuel cells are less noisy and less cause vibration. The fuel cells are therefore promising as energy sources covering a variety of applications and scales. Specifically, the fuel cells have been developed in forms of polymer electrolyte fuel cells (PEFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and biofuel cells. In particular, the polymer electrolyte fuel cells have been developed for use in fuel cell vehicles, domestic cogeneration systems, and mobile devices such as mobile phones and personal computers.
Such a polymer electrolyte fuel cell is hereinafter also simply referred to as a “fuel cell”. The fuel cell includes a plurality of unit cells, each unit cell including an anode, a cathode, and a polymer electrolyte membrane disposed between the anode and the cathode, where the unit cells are stacked together with separators therebetween. The separators have grooves as flow channels for a gas such as hydrogen or oxygen. Such separators are also called bipolar plates.
The separator also acts as a component that leads a generated current from the fuel cell to the outside. A material having a low contact resistance is therefore used for the separator, where the contact resistance refers to a voltage drop due to an interfacial phenomenon between the electrode and the separator surface. In addition, the separator also requires high corrosion resistance, because the inside of the fuel cell is in an acidic atmosphere at a pH of about 2 to about 4. The separator also requires certain durability to maintain the above-described low contact resistance over a long duration during use in such acidic atmosphere. Thus, there have been used carbon separators that are milled from graphite powder compacts, or molded from a mixture of graphite and a resin. The carbon separators, however, are inferior in strength and toughness and may be broken when vibration or an impact is applied to the fuel cell. Hence, investigations have been made on separators made of metal materials having excellent workability and high strength, such as aluminum, titanium, nickel, alloys based on these metals, and stainless steels.
When a metal material such as aluminum or stainless steel is used for the separator, the material is corroded due to the inside acidic atmosphere of the fuel cell. This results in elution of metal ions, leading to early degradation of a polymer electrolyte membrane and a catalyst. In contrast, when a metal having high corrosion resistance, such as titanium, is used, a passive film is formed in a corrosive environment. The separator in this case has inferior (increased) contact resistance, because the passive film has low conductivity. Under these circumstances, there has been developed a separator including a substrate made of a metal material and a coating over the surface of the substrate, where the coating has certain conductivity that can be maintained over a long duration so as to add high corrosion resistance and high conductivity to the substrate.
Materials for the coating having high conductivity and still high corrosion resistance include noble metals such as Au and Pt or alloys of such noble metals. However, a separator, if coated with such a noble metal material, suffers from high cost. In a previously disclosed technology of a separator, a coating containing a carbon material is used as an inexpensive material having certain conductivity and corrosion resistance. The coating is disposed over the metallic substrate. Typically, each of separators disclosed in Patent literature (PTL) 1 and PTL 2 includes a substrate and carbon particles dispersed over the substrate. The substrate is composed of an austenite or austenite/ferrite duplex stainless steel being combined with Cr and Ni and having particularly high acid resistance among stainless steels, and the carbon particles are applied and brought into intimate contact with the substrate surface by compression bonding (PTL 1) or heat treatment (PTL 2).
In these separators, however, the carbon particles adhere to the substrate in islands, and the substrate is partially exposed. Even if the substrate is composed of the stainless steel having high acid resistance, iron ions may be eluted during use in a fuel cell. To prevent this, previously-developed separators employ highly corrosion-resistant titanium as a substrate. These separators are free from the risk of corrosion of the substrate regardless of environmental barrier properties of a conductive coating on the surface.
Independently, PTL 3 discloses a separator that is prepared by depositing a carbon film on a substrate at a high temperature through chemical vapor deposition (CVD) or sputtering to cover the substrate surface. The resulting carbon film becomes amorphous, has higher conductivity, and allows the separator to have low contact resistance. PTL 4 discloses a separator that includes a metal substrate, a conductive thin film disposed over the substrate surface and including carbon, and an intermediate layer disposed between the substrate and the conductive thin film. The metal substrate bears an oxide layer that is allowed to remain for better corrosion resistance. The intermediate layer is disposed so as to offer adhesion between the oxide layer and the conductive thin film and includes a metal element selected from elements such as Ti, Zr, Hf, Nb, Ta, and Cr, or a metalloid element such as Si. In addition, the separator has a gradient mixing ratio of the metal or metalloid element to carbon from the intermediate layer to the conductive thin film at an interface between them. PTL 5 discloses a separator that includes a metal substrate, a diamond-like carbon layer over the substrate, and a conductive section over the diamond-like carbon layer. The conductive section includes graphite particles dispersed and disposed over the diamond-like carbon layer. The diamond-like carbon layer and the conductive section are formed with an arc ion plating (AIP) apparatus so as to impart corrosion resistance to the substrate.
The separators disclosed in PTL 3 to 5 each have an amorphous carbon film deposited by CVD or sputtering, thereby have insufficient barrier properties, suffer from deterioration in conductivity though the substrate being protected by the passive film from corrosion, and have insufficient adhesion between the carbon film and its underlying layer (the metal substrate or intermediate layer). In addition, the process such as CVD requires a long time to deposit the carbon film to a sufficient thickness. Thus, the separators are inferior in productivity.
Under these circumstances, some of the inventors have developed a separator including a titanium substrate, a carbon layer over the substrate, and an intermediate layer between the substrate and the carbon layer. The intermediate layer includes titanium carbide and is formed by a heat treatment after forming the carbon layer over the substrate (see PTL 6). The intermediate layer formed in the above manner by a reaction between titanium of the substrate and carbon of the carbon layer provides excellent adhesion between the carbon layer and the substrate. In addition, conductive titanium carbide formed in the intermediate layer allows the separator to have low resistance.